Investigating ChaMPlane X‐Ray Sources in the Galactic Bulge with Magellan LDSS2 Spectra

Abstract

We have carried out optical and X-ray spectral analyses on a sample of 136 candidate optical counterparts of X-ray sources found in five Galactic bulge fields included in our Chandra Multiwavelength Plane Survey. We use a combination of optical spectral fitting and quantile X-ray analysis to obtain the hydrogen column density toward each object, and a three-dimensional dust model of the Galaxy to estimate the most probable distance in each case. We present the discovery of a population of stellar coronal emission sources, likely consisting of pre-main-sequence, young main-sequence, and main-sequence stars, as well as a component of active binaries of RS CVn or BY Dra type. We identify one candidate quiescent low-mass X-ray binary with a subgiant companion; we note that this object may also be an RS CVn system. We report the discovery of three new X-ray-detected cataclysmic variables (CVs) in the direction of the Galactic center (at distances less than or similar to 2 kpc). This number is in excess of predictions made with a simple CV model based on a local CV space density of less than or similar to 10(-5) pc(-3), and a scale height similar to 200 pc. We discuss several possible reasons for this observed excess.